Remove fewer Storage calls in copy_prop

[ohadravid]

Modify the copy_prop MIR optimization pass to remove fewer Storage{Live,Dead} calls, allowing for better optimizations by LLVM - see #141649.

Details

This is my attempt to fix the mentioned issue (this is the first part, I also implemented a similar solution for GVN in this branch).

The idea is to use the MaybeStorageDead a new MaybeUninitializedLocals analysis and remove only the storage calls of heads that are maybe-uninit when the associated local is accessed (or, conversely, keep the storage of heads that are for-sure initialized in every relevant access).

When combined with the GVN change, the final example in the issue (#141649 (comment)) is optimized as expected by LLVM. I also measured the effect on a few functions in rav1d (where I originally saw the issue) and observed reduced stack usage in several of them.

This is my first attempt at working with MIR optimizations, so it's possible this isn't the right approach — but all tests pass, and the resulting diffs appear correct.

r? tmiasko

since he commented on the issue and pointed to these passes.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[matthiaskrgr]

@bors try @rust-timer queue

[bors]

⌛ Trying commit d24d035 with merge ef7d206...

[bors]

☀️ Try build successful - checks-actions
Build commit: ef7d206 (ef7d20666974f0dac45b03e051f2e283f9d9f090)

[rust-timer]

Finished benchmarking commit (ef7d206): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.4%]	8
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.2%, 0.4%]	8

Max RSS (memory usage)

Results (primary 0.7%, secondary 3.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.5%	[1.8%, 5.0%]	5
Regressions ❌ (secondary)	3.4%	[3.4%, 3.4%]	1
Improvements ✅ (primary)	-3.9%	[-6.5%, -2.0%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[-6.5%, 5.0%]	8

Cycles

Results (primary -0.6%, secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.8%	[3.8%, 3.8%]	1
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-4.1%	[-4.1%, -4.1%]	1
All ❌✅ (primary)	-0.6%	[-0.6%, -0.6%]	1

Binary size

Results (primary 0.0%, secondary 0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.8%]	10
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	5
Improvements ✅ (primary)	-0.2%	[-0.8%, -0.0%]	8
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	0.0%	[-0.8%, 0.8%]	18

Bootstrap: 757.399s -> 756.065s (-0.18%)
Artifact size: 372.20 MiB -> 372.12 MiB (-0.02%)

[ohadravid]

@matthiaskrgr - I updated the impl to stop re-checking once a head is found to be maybe-dead, which should be a bit better

[matthiaskrgr]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 905e968 with merge c0a2949...

[cjgillot]

Should this check happen in Replacer::visit_local, and move the replacement of storage statements to a dedicated cleanup visitor?

[bors]

☀️ Try build successful - checks-actions
Build commit: c0a2949 (c0a294957df10fc3880e1677c72c0cf122485509)

[ohadravid]

Should this check happen in Replacer::visit_local

I'm not sure how to make this work: using ResultsCursor requires a &body, but it's not possible to have that while running a MutVisitor since it requires a &mut body.

Is there a different way to do this?

[rust-timer]

Finished benchmarking commit (c0a2949): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.4%]	9
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	0.3%	[0.2%, 0.4%]	9

Max RSS (memory usage)

Results (primary -0.1%, secondary -1.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	4.2%	[3.4%, 5.8%]	4
Regressions ❌ (secondary)	3.1%	[3.1%, 3.1%]	1
Improvements ✅ (primary)	-4.4%	[-6.6%, -1.8%]	4
Improvements ✅ (secondary)	-5.8%	[-5.8%, -5.8%]	1
All ❌✅ (primary)	-0.1%	[-6.6%, 5.8%]	8

Cycles

Results (secondary -1.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.6%	[-2.6%, -2.5%]	2
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.0%, secondary 0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.8%]	10
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	5
Improvements ✅ (primary)	-0.2%	[-0.8%, -0.0%]	8
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-0.0%	[-0.8%, 0.8%]	18

Bootstrap: 756.494s -> 757.685s (0.16%)
Artifact size: 372.15 MiB -> 372.11 MiB (-0.01%)

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

tests/pass/float_nan.rs ... ok
tests/pass/0weak_memory_consistency_sc.rs ... ok

FAILED TEST: tests/pass/rc.rs (revision `stack`)
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-Kr485W" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/miri" "--error-format=json" "--sysroot=/checkout/obj/build/x86_64-unknown-linux-gnu/miri-sysroot" "-Dwarnings" "-Dunused" "-Ainternal_features" "-O" "-Zmir-opt-level=4" "-Cdebug-assertions=yes" "-Zui-testing" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/tmp/miri_ui/0/tests/pass" "tests/pass/rc.rs" "-Zmiri-strict-provenance" "--edition" "2021" "--cfg=stack" "-Cextra-filename=stack"

error: test got exit status: 1, but expected 0
 = note: compilation failed, but was expected to succeed

full stderr:
---
   = note: inside `rc_fat_ptr_eq` at tests/pass/rc.rs:76:26: 76:27
note: inside `main`
  --> tests/pass/rc.rs:144:5
   |
LL |     rc_fat_ptr_eq();
   |     ^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to 1 previous error; 2 warnings emitted


full stdout:



FAILED TEST: tests/pass/rc.rs (revision `tree`)
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-Kr485W" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/miri" "--error-format=json" "--sysroot=/checkout/obj/build/x86_64-unknown-linux-gnu/miri-sysroot" "-Dwarnings" "-Dunused" "-Ainternal_features" "-O" "-Zmir-opt-level=4" "-Cdebug-assertions=yes" "-Zui-testing" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/tmp/miri_ui/0/tests/pass" "tests/pass/rc.rs" "-Zmiri-strict-provenance" "-Zmiri-tree-borrows" "--edition" "2021" "--cfg=tree" "-Cextra-filename=tree"

error: test got exit status: 1, but expected 0
 = note: compilation failed, but was expected to succeed

full stderr:
---
   = note: inside `rc_fat_ptr_eq` at tests/pass/rc.rs:76:26: 76:27
note: inside `main`
  --> tests/pass/rc.rs:144:5
   |
LL |     rc_fat_ptr_eq();
   |     ^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to 1 previous error; 2 warnings emitted
---

Location:
   /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ui_test-0.29.2/src/lib.rs:369

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
error: test failed, to rerun pass `--test ui`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/release/deps/ui-3ca614ea6d3a8322 tests/pass tests/panic` (exit status: 1)
Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:05:41
  local time: Wed Jun 18 08:12:15 UTC 2025
  network time: Wed, 18 Jun 2025 08:12:15 GMT
##[error]Process completed with exit code 1.
Post job cleanup.